1. Field of the Invention
This invention relates to a method for forming a thin film and an apparatus for forming the thin film, and in particular, to a method for forming a thin film of a uniform thickness.
2. Description of the Related Art
FIG. 8 is a cross sectional view, showing a vertical type LPCVD (Low Pressure Chemical Vapor Deposition) reactor which uses a conventional thin-film-forming method. The vertical type LPCVD reactor is used in a process for manufacturing semiconductor devices, to form silicon oxide films, polycrystal silicon films, or the like on a plurality of semiconductor substrates.
As is shown in FIG. 8, a reactor 2, which comprises first and second quartz tubes 3 and 4, is provided inside a cylindrical heater 1. The first quartz tube 3 is located near the, inner wall of the heater 1, having one end sealed and the other end connected to one end of a manifold 5. The other end of the manifold 5 is sealed with a sealing member 6. First and second gas nozzles 7 and 8 and an exhaustion port 9 are located at peripheral portions of the manifold 5. One end of the second quartz tube 4 is located near the inner surface of the manifold 5. The other end of the quartz tube 4 is placed in the vicinity of the sealed end of the first quartz tube 3. Thus, the second quartz tube 4 is set inside the first quartz tube 3.
The first gas nozzle 7 has one end located inside the other end of the second quartz tube 4. A plurality of gas supply ports 7a are formed in the gas nozzle 7, extending toward the axis of the second quartz tube 4. The second gas nozzle 8 has one end located inside the second quartz tube 4 and near the one end thereof.
A heat retaining cylinder 10 is arranged above the sealing unit 6 and inside the second quartz tube 4. A boat 12 for containing a plurality of wafers 11 is located above the heat retaining cylinder 10.
In operation, wafers 11 are placed in the boat 12, which is inserted into the reactor 2. Thereafter, the reactor 2 is heated by the heater 1 to a predetermined temperature. Then, a material gas (not shown) is introduced into the reactor 2 through the first and second gas nozzles 7 and 8, and applied to the wafers 11. The gas flows in the directions of arrows 13. On the wafers 11, the gas is decomposed, forming thin films thereon. During forming the films, the surface temperature of the wafers 11 is kept almost constant to obtain films of a predetermined thickness. Specifically, changes in surface temperature is held at 1.degree. C. or less.
Although in the conventional thin-film-forming method, the surface temperature of the wafers 11 are maintained almost constant to form films of a uniform thickness, uniform films cannot be obtained in some cases.
If, for example, P-doped SiO.sub.2 films are formed on the wafers 11 in the vertical type LPCVD reactor shown in FIG. 8, using TEOS (tetraethyl orthosilicate) and PH.sub.3 gases as material gases, at an internal reactor temperature of 600.degree. C. and under an internal reactor pressure of 0.5 Torr, the SiO.sub.2 films will have the thickness distribution as shown in FIG. 9. As is evident from FIG. 9, each wafer is thicker at the peripheral portion than at the central portion.
Further, if P-doped polycrystal silicon films are formed on the wafers 11 by the vertical type LPCVD reactor, using SiH.sub.4 and PH.sub.3 gases as material gases, at an internal reactor temperature of 600.degree. C. and under an internal reactor pressure of 0.5 Torr, the polycrystal silicon films will have the thickness distribution as shown in FIG. 10. Obviously, each wafer is thicker at the peripheral portion than at the central portion, as the SiO.sub.2 film.
If the thickness of a thin film formed on a wafer is not uniform, wiring contact may fail at a thick portion of the wafer, or the workability of the wafer be reduced. Consequently, a semiconductor device formed on the wafer may have but low operability.
FIG. 11 shows an essential part of a conventional thin-film-forming apparatus. This apparatus differs from the conventional vertical type LPCVD reactor shown in FIG. 8 only in that interrupting flat rings 15 are attached to the boat 12 for enhancing the uniformity of the thickness of a thin film.
Each of the flat rings 15 is interposed between peripheral portions of each adjacent pair of the wafers 11 contained in the boat 12. The rings 15 interrupt the material gases from flowing between the adjacent wafers. This makes it possible to form a thin film of a uniform thickness on each wafer 11.
FIG. 12 shows an essential part of another conventional thin-film-forming apparatus. The apparatus differs from the conventional vertical type LPCVD reactor shown in FIG. 8 only in that interrupting flat disks 16 are attached to the boat 12 for enhancing the uniformity of the thickness of a thin film.
Each wafer 11 is placed on a corresponding one of the flat disks 16 which have a diameter larger than the, wafer 11. A thick film is thereby formed on a peripheral portion of the flat disk 16. As a result, a thin film of a uniform thickness is formed on each wafer 11.
As explained above, a uniform thin film can be formed on each wafer 11 in the conventional thin-film-forming apparatuses. However, the use of the interrupting rings 15 or disks 16 reduces the number of wafers 11 that can be treated at a time, ultimately increasing the manufacturing cost of resultant semiconductor devices. Further, an apparatus provided with such rings or disks is complicated in structure. Inevitably, its manufacturing cost is high, and its maintenance is difficult.